This invention relates to a method for the recovery of vapors of liquid hydrocarbons or other chemicals, and more particularly to the recovery of vapors which are displaced from tanks during the filling of the tank with liquid hydrocarbons or other chemicals.
As liquid hydrocarbons or other volatile chemicals are loaded into storage tanks such as on ships, trucks or railroad cars, a portion of the hydrocarbon or chemical enters the vapor phase and mixes with air and other vapors present in the tank. The vapor mixture is then expelled from the tank as the liquid level in the tank rises. Although the separation and recovery of the hydrocarbon or other chemical from the vapor mixture is often economically unfeasible because of the high equipment and operating costs of conventional recovery processes, the expelled vapor is a significant contributor to air pollution and federal legislation has been enacted which strictly limits the release of such vapors.
Previous attempts to recover hydrocarbon vapors displaced during the filing of storage tanks have utilized large condensation or absorption columns in combination with compressors and/or other apparatus to recover the hydrocarbon vapor as a liquid. Examples of such processes are disclosed in U.S. Pat. Nos. 3,967,938, 4,110,091, 3,886,759, and 3,714,790.
Another type of system for controlling hydrocarbon and other emissions during filling of storage tanks utilizes activated carbon or other solid adsorbent beds which selectively adsorb the hydrocarbon vapors from the displaced vapor stream. The adsorbent bed is regenerated such as by a reduction in pressure to desorb the hydrocarbon from the activated carbon. The hydrocarbon stream is then combusted or further processed in an absorption tower to recover a portion of the hydrocarbon as a liquid. An example of such a system is disclosed in U.S. Pat. No. 4,066,423.
In the vapor handling systems described above, the presence of air in the incoming vapor mixture presents a significant risk of combustion of the hydrocarbon vapor. To reduce the risk of combustion, a liquid hydrocarbon stream is often provided to saturate the vapor mixture. Specialized equipment such as flame arrestors and liquid ring pumps may also be utilized in an attempt to reduce the risk of fires or explosion. Water vapor in the incoming vapor mixture likewise presents considerable processing difficulties as the low temperatures required to separate the hydrocarbon from the mixture leads to freezing of the water vapor on system components such as heat exchanger tubes. The entire system must be frequently be shut down or backup equipment provided so that the frozen water may be periodically removed from such surfaces in order to maintain operability of the system. Yet, despite the high energy and specialized equipment requirements of such vapor control systems, a vapor overhead containing up to 10% hydrocarbon vapor may still be released to the atmosphere from the condensation tower overhead of at least one such system.
Vapor control systems such as those described above can be installed at loading terminals and connected to the tanks being loaded by a series of connecting pipes and blowers which direct the vapors from the tanks to the vapor control system. Because of the high equipment and operating cost for such systems, a single vapor control system is typically utilized for controlling vapors from a plurality of loading tanks. The use of a single system for multiple loading tanks, however, presents a difficult engineering problem since the quantity of vapor handled by the system will vary widely depending upon the number of tanks being filled at a given moment. In addition, the piping used to connect the vapor control system with the loading tanks is a potential source of leakage and resulting fire or explosion, particularly at marine terminals where the ship or barge may move in the water and disrupt the connecting vapor control pipes.
Inert gases have also been used in an attempt to reduce the risk of fire or explosion from hydrocarbon vapor emissions during loading operations. In one process, a flue gas comprising principally carbon dioxide, nitrogen and water vapor is introduced into the loading tank and mixed with the hydrocarbon vapors to provide an inert vapor mixture which is then vented directly to the atmosphere. While this type of vapor control system reduces the risk of fire or explosion, it was notably deficient in that the vented flue gas and hydrocarbon vapor mixture serve as a source of air pollution.
In another vapor control process, a nitrogen inerting gas is introduced into the holding tank of a barge or similar vessel to provide an inert vapor mixture in the vapor space above the hydrocarbon liquid during transport of the barge to its destination. During unloading of the hydrocarbon liquid from the barge, the nitrogen and hydrocarbon vapor mixture which has formed in the vapor space is directed onshore where the nitrogen vapor is separated from the vapor mixture and vented to the atmosphere. While this type of process is beneficial in view of the reduced risk of fire or explosion during transport and unloading of the barge, the inert gas is not recovered and a new source of nitrogen must be provided each time the barge is loaded.